This invention relates to a liquid filtering device for removing solid contaminants from liquid such as oil. Conventional liquid filters can be classified into two types, one utilizing surface layer filtering action and the other deep layer filtering action. According to the former, the contaminants in the liquid are trapped at and removed by the surface of a filter material, whereas according to the latter, the contaminants are trapped by the wall of the capillary passages, interstices or small voids in the filter material. With the surface layer filtering, as the contaminants deposit on the surface of the filter material the passages through the material are clogged by the deposited contaminants with the result that the resistance against the flow of the liquid increases gradually. For this reason, the filter material is required to have sufficiently large surface area.
On the other hand, with the deep layer filtering, as all wall surfaces of small voids or capillary tubes are efficiently used to trap the contaminants, it is possible to trap a large quantity of contaminants even with a filter of relatively small volume. However, in the filter of this type too, the liquid passages will be ultimately clogged by the trapped contaminants. Further, when mounting a filter material having a substantial thickness in the casing of a filtering device it is difficult to maintain at a sufficiently liquid tight condition the contact surface between the filter material and the casing.
For this reason, according to an established theory of filtering, with a filtering material of a definite volume, it is possible to treat only a definite volume of the liquid under prescribed conditions so that it is necessary to frequently clean or renew the filter material.
In certain cases, a primary filter unit for removing relatively coarse solid contaminants, and a secondary filter unit for removing relatively fine solid contaminants are connected in series in the same tank. In such an installation, the primary filter unit will be relatively quickly clogged by the trapped contaminants so that it is necessary to frequently renew the primary filtering unit. Accordingly, it is desirable to provide means for determining the degree of clogging or the remaining filtering capability of the primary filter unit to enable exchange thereof at a proper time.